Infant siblings of children with ASD exhibit substantial variability in their social-communicative outcomes: some manifest behaviors consistent with an autism spectrum diagnosis, others exhibit less severe symptoms such as language delay, and others evidence no detectable disorder. This cross-site, Vanderbilt- Miami project will examine the extent to which early impairments in specific attentional and affective mechanisms - both of which are putative core deficits of autism - explain the behavioral heterogeneity that is observed in these high-risk children. We focus specifically on the domains of attention coordination and positive affective competencies, and examine their development across the first two years of postnatal life (i.e., from 6 to 18 months) and their relation to later social and communicative outcomes at age 24 and 36 months. The overarching hypotheses are that there will be developmental continuity in these domains across the first two years, that early differences in these domains will be evident between groups at high- and low-risk for ASD, and that early impairments will predict individual differences in social and communicative outcomes observed in later-born ASD siblings. Specific areas of investigation include the relation between positive affective competencies in dyadic and triadic contexts across the first year of life, the contributions of early positive affective competencies and attention coordination to the emergence of positive joint attention competencies in the second year of life, and the extent to which individual differences in early positive affective competencies and attention coordination contribute to later autism symptomatology. This prospective longitudinal project will compare developmental trajectories and outcomes in 200 infant siblings of children diagnosed with ASD (Sib-ASD) and 100 infant siblings of typically developing children (Sib-TD). A split panel longitudinal design will allow us to examine development both within- and across- age-group cohorts from 6 months to 36 months of age. This project employs novel methods and measurement of infant behavior including electrophysiological and eye tracking measures, measures of positive affect communication and representation, and longitudinal measures of sharing positive emotion about objects with others. Longitudinal structural equation modeling and mixed modeling procedures are utilized to assess differences in the development of typically developing and at- risk siblings and to predict heterogeneity in the outcomes of the at-risk siblings of children with ASD. The project employs a developmental psychopathology perspective to shed light on normative and disturbed longitudinal pathways toward heterogenous outcomes. PUBLIC HEALTH RELEVANCE: The prospective study of younger siblings of children with ASD offers an ideal opportunity to learn about the earliest signs of autism as well as the development, manifestation, and boundaries of the broader autism phenotype. This information is critical for understanding the causes of autism and for developing more effective and targeted treatments.